As a component in a photovoltaic power generation system, a grid-connected photovoltaic inverter is used to convert direct current (DC) from solar panels into alternating current (AC) which has the same phase and the same frequency as grid voltage, and to feed the AC to a power grid. When the power grid interrupts power supply for some reason, the inverter still transmits electrical energy to the grid. The inverter and local load form a self-supply island that cannot be controlled by a public grid system, which is also known as an islanding phenomenon. If there is no islanding detection or protection technique, the inverter will provide power supply to the local load and a local grid load continuously. The grid-connected photovoltaic (PV) power generation system in an islanding running state will result in serious consequences including: the power grid cannot control the voltage and frequency of the island; if the voltage and frequency exceed permitted ranges, it may cause damage to users' devices; if the load capacity is larger than the capacity of the PV power generation system, the PV power generation system will be in an overload running state which can be easily burned; live lines connected with the PV power generation system will decrease the security of the grid and are dangerous to maintenance personnel; reclosing operation of the island will cause line tripping and may cause damage to the PV power generation system and other equipments. It can be seen that inverters that are designed to have fast and accurate response function of islanding detection can reduce the harm of islanding.
Current islanding detection methods mainly include passive detection method and proactive detection method. Passive detection is to detect islanding effect by the variation of the voltage, frequency, phase or harmonic wave at the output of the inverter when the grid interrupts supply. This detection method determines islanding phenomenon mainly by an over/under-voltage and over/under-frequency detection method, and stops the inverter's grid-connected operation when a voltage-amplitude and a frequency at the node of common coupling (PCC) exceed normal ranges. When the inverter is operating, operational ranges of the voltage and the frequency should be reasonably set. If a voltage or a frequency offset reaches a preset threshold of the islanding detection, an islanding phenomenon may be detected. However, when the local load of the inverter closely matches the output power of the inverter, the offsets of the voltage and the frequency are very small or even zero, therefore, the passive detection method has a large non-detection zone (blind zone) and it fails for systems with good load matching.
The proactive detection method is to control the inverter to make certain disturbances to its output power, frequency or phase. When the grid is in normal operation, these disturbances can not be detected because of the balancing effect of the grid. Once the grid fails, disturbances output from the inverter accumulate quickly and exceed permitted ranges, thereby triggering islanding detection circuit. Such a method, which has a high detection accuracy and a small non-detection zone, requires complicated control and reduces quality of the power from the inverter.